High-frequency treating system for dielectric materials



Apil 25, 1950 w. ATwooD HIGH-FREQUENCY TREATING SYSTEM FOR DIELECTRIC MATERIALS 2 Sheets-Sheet l Filed June l, 1945 R. o Mw n [T A MVA L. m VM m .W/ n.. Ww TS.

April 25', 195() w. L. ATwooD HIGH-FREQUENCY TREATING' SYSTEM FOR DIELECTRIC MATERIALS 2 Sheets-Sheet 2 Filed June l, 1945 Y R f m0 M uw m WW M7 L. j m .W/

Patented pr. 25, 1950 HIGH-FREQUENCY .TREATINGSXSTEMFOR DELEGTRIC MATERIALS Wilfrid L. Atwood, Jeierson-County,

Ky., as-

signor to 1 The Girdler Corporation, Louisville, Ky., a corporation of Delaware Application June l, 1945,v Serial No. 597,048

3 Claims.

'This invention relates to high-frequency treating systems for dielectric materialsand has for an. object the vprovisionfoffmeans for automatically controllingthe' load or the amount of electrical poweror'energy applied'to the dielectric material undergoing treatment.

It has heretofore beenrecognized that dielectricv materials change their characteristics with increasing temperature.- For many materials the power factor and dielectric constant of such materials increase as the temperature rises. It has, therefore, .been suggested that a variable inductor be included in the load lcircuit to maintain it ata frequency resonant with the frequency from the high-frequency generator. The adjustment ofthe-variable inductor-.would be in a direction toicompensate for thevchange in the dielectric constant, or capacitance, d-ue to the rising temperature of the dielectric material.

Also, it hasbeen-suggestedg-.to include a variable coupling between g the f. generator r`and the load circuit, which coupling may be adjusted to compensate for changes irl-power factor resulting from changes in temperature of the dielectric material. The disadvantage with'systems of this type is that two variables are changed, in that there is'a change in the-load and ay change in the resonant frequency.

In the application filed concurrently herewith, Serial Number 597,047, by Paul D. Zottu, a co-worker of mine, there isL disclosed a system in which the dielectricv load is directly included in the tankcircuitl as capacity-together witha grid circuit energized directly from the tank circuit. My inventionis particularly. applicable to high-frequency generating systems of this type, though certainaspects thereoimay be applied to other electrical systems.

n Inv carryingaout the present vinvention in one form thereof, ther-'eis provided-a high-frequency generator or oscillator havingan-output circuit which formsthe `tank circuit-.of the oscillator. The capacitance between the electrodes to which the high-frecluerlcy electrical energy is applied is automatically adjusted -tomaintain-a predef terminedload on the-'high--frequency oscillator or generator. 'Morespecically thespacing of the electrodes jis adjustedso as to vmaintain ata predetermined value the, anodeorplate currentof the oscllatorandgpreferably toY mtaintain it at substantially the normal rated load thereof. This is accomplished by means of a device sensitive to small changes in anode current and which through electric valve means controls the operation of a motor to adjust the spacing of the electrodes in the correct `direction to maintain the anode current at-its predetermined value.

For a more complete understanding of the invention and foriurther objects and advantages thereof, reference should nowV be hadto the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a wiring diagram which schematically illustrates a system embodying the inventori;

Fig. 2 diagrammatically illustrates one suitable-mechanism for adjustably supporting one of the electrodes; and

Fig. 3 diagrammatically illustrates a system in which electric valves of a different type have been utilized.

Referring now toFig. 1, there has been illustrated apair of'electrodes Iii-and II, between which there is-disposed -dielectric material I2 to be treatedfor-heated by the application thereto of high-frequency electrical energy. High-frequency electrical energy -is generated byan oscillator which includes electric valve means I3 having an input circuit which may be traced from a rectier I4 by way of a choke coil I5, the anode of the valve means I3, thence to the cathode and from the mid-tap of the secondary winding of a lament transformer I6 to ground. The return circuit is from ground .by way of variable resistor Il, an overload relay I9, and by conductor 29 to the other side of the rectier I4.

The electrical generator or oscillator includes a tank circuit connected by way ofthe blocking capacitor 2l to the anode of the valve means I3 and .which comprises an inductor 22, conductor 23, a movable support or post 2li which carries the electrode Il), .the capacitance between the electrodes Ii) and II. The electrode II is connectedjby the ground connection to the other sideof the inductor 22. The generator includes a grid circuit which by means of the inductor 25 is inductively coupled to the inductor 22. This grid circuit includes the grid in the valve means I 3, and in this grid circuit there is included a resistor 26 shunted by av capacitor 21 to form a grid-biasing means. The grid circuit isconnected to the mid-tap of the secondary of the filament transformer IB.

It will be understood that upon energization of the filament transformer I6 from a suitable source of s-upply indicated at 3l and 32, and upon energization of the rectier I4, the generator will supply electrical energy to the dielectric material i2 with a frequency determined by the value of the inductance oi inductor 22 and the value of the capacitance between electrodes Iii and II. In general, these values will be selected so that the resonant frequency of this tank circuit will ordinarily lie in the range of from 1 megacycle to 3l) or more megacycles, though frequencies of other ranges may be utilized. Since the grid circuit is energized directly from the tank circuit which includes the load, or the dielectric material I2, it will be understood that the frequency of the energy supplied to the grid circuit will vary in accordance with any change in the capacitance between the electrodes It and II. A change in the capacitance of the tank circuit produces a change in the resonant frequency thereof. The excitation of the grid circuit likewise changes. There is no detuning of the one circuit with respect to the other, with a consequent reduction in load. The load on the oscillator will, therefore, be largely dependent upon the voltage applied to the load and this voltage varies with the spacing of the electrodes. In consequence, the load may be regulated directly by changing the spacing between the electrodes I and Il, which, of course, changes the capacitance oi' the output tank circuit. Accordingly, if this capacitance is adjusted in proper manner, the generator may be operated at any desired load, or at the op timum load for the valve means I3.

In accordance with the invention, automatic control is accomplished by adjusting the spacing of the electrodes in accordance with changes in value of the anode or plate current. The manner in which this is accomplished will be fully explained, but before doing so, reference will first be had to the system as a whole, which system includes certain desirable features utilized for the general protection of circuit elements.

In starting up the system, the main line switch or circuit breaker is closed to apply alternating current to supply lines 3| and 32. It will be observed that the operating coil of a relay 33 is immediately energized. This relay may be ci any conventional design, of which there are a number on the market, having the characteristic or closing its contacts a predetermined time interval after energization of the operating coil. As indicated by the reference characters applied to filament transformer i6, as soon as the circuit breaker Sii has been closed, the primary winding or the filament transformer it is energized to initiate heating of the lament or cathode oi the valve means I3. After a time interval adequate for the cathode to be heated to a desired operating temn perature, the relay 33 closes its contacts to complete an energizing circuit for a relay This circuit may be traced from supply line 3i by way of a slide-actuated switch 36, the conductor s?, closed contacts of the relay 33, the operating coil of relay 35, the closed contacts ci a timing relay 38 and by the normally closed contacts of the overload relay I9 to the other supply line 32. Accordingly, the relay 35 is energized to close its contacts 3Q and li. The closure of the contact 39 completes a holding circuit for the operating coil of relay 35 which may be traced by way of l tacts lili.

e conductor 4I, contacts 39, conductor 3l, contacts of relay 33 and thence through the remaining part of the circuit traced above.

It may be further observed that as soon as the circuit breaker 3Q was closed, an energizing circuit was completed for the operating coil of a contactor 42 which may be traced from the supply line 3i by way of conductor 43, contacts ifi of a relay dii, the operating coil of contacter 52, and by the contacts or a limit switch fr@ to the other supply line S2. In consequence, the contactor closes its contacts to energize from a suitable source of supply, such as indicated at te, a motor for operation thereof in a direction to raise the t electrode II] to its uppermost position. As shown,

the motor is drives a gear et which meshes with a rack gear 5l carried by the post for the raising and lowering of electrode iii. As the electrode I0 and the post 24 move upwardly, an insulated extension 24a has an inwardly extending projection .ib which operates the contacts i5 of the limit switch to interrupt the circuit traced therethrough. Thus, the motor :i9 is de-energized as soon as the electrode I0 has been moved to its uppermost position. rIhe electrode ill is retained in that position, since the motor must be energized for movement of the electrode Il) in either direction. With the electrode I0 in its uppermost position, a minimum of capacitance is included in the output or tank circuit of the high-frequency generator.

it has been found convenient to insert dielectric materials between the plates Iii and Ii by utilizing a slide or drawer which, when disposed symmetrically between the electrodes Ill and II, closes the slide-actuated switch 3S. Hence, it will now be assumed that the slide has been moved to its operating position so that the switch Sii moves from the illustrated position to one in which a circuit is completed by way of conductor 55 for the operating coil of the relay 45. This circuit may be traced from the other side of such operating coil to the other side s2 of the line by way of the contacts te of relay 35.

i, The relay or contactor thereupon closes to energize through its contacts 56 and 5I the trans former 5S which supplies the rectier and the anode circuit of the valve means I3. t the same time, it opens its contacts 45 closes its conw Inasmuch as the rectifier is now energized to apply plate or anode voltage to the valve means I3, oscillations are immediately initiated and high-frequency electrical energy is supplied to the electrodes Ill and I I. However, since there is a substantial spacing between them, the oscillator is not loaded to a very great degree, though it may be o1- 75 of full load, depending upon the position selected for the limit switch it.

In order to increase the power or energy supf plied to the load I2, the spacing of the electrodes Ill and I I is decreased. This is accomplished as a function of the potential difference which exists across the resistor I'I, which it will be remembered is included in the anode supply circuit. The potential difference or IR drop across the resistor II is applied by conductors G2 and 63 to a sensitive device shown in the form of a contactmaking galvanometer B4. The galvanometer 34 has a pointer or contact-making member operable between contacts 66 and 51. As already explained, in starting up the oscillator, the spacing between the electrodes I!) and II is a maximum, and the anode current is small. Therefore, the sensitive device B4 will not be energized suiciently to have moved the contact member S5 acoge als away from the contact 66. Accordingly, a relay 60 will be de-energized for completion of a circuit for the operating coil of a contactor 69 which controls the energization of the motor 49 for rotation in a direction to lower the electrode l0.

While the sensitive device 64 might, from a theoretical standpoint, be utilized directly to controy relays for the operation of the contactors l2 and S9, from a practical standpoint the operation would very likely be erratic. This arises from the fact that positive operation is desired in response to relatively small changes in value of the anode current, for example, operation is desired in response to a change in the anode current of the ordey` of 2% of the full load current which, in the absence of the invention, might s arise due to erratic contact resistance between the contact member 65 and the contacts 66 and 61. More specifically, a pair of electric valves and 1i are provided for positive control of the relay @il and a relay 12 provided to control the ip energization of the contactor 32. The valves 10 and 1l have their anode circuits connected to the secondary winding of a transformer 13 supplied from a suitable source of alternating current, as for example from the conductors 3! and 32. The anode circuit for the valve 10 may be traced from one side of the secondary winding of transformer 13 by way of the operating coil of relay 12, the anode and cathode of valve 10, the resistor 14 and by conductor 15 to the other side of said secondary winding. Similarly, the the operating coil of the relay S3 is included in the anode circuit of the valve 1l, and a resistor 16 is included in circuit with the cathode-return circuit thereof. The operating coils of relays 53 and 12 are by-passed or shunted by capacitors 'i1 and 18. These capacitors function to maintain the relays in their energized positions, notwithstanding the fact that half-wave unidirectional energizing current is supplied thereto by the valves 10 and 1l. The resistors 19 and 8E are respectively connected between the grids and cathodes of the respective valves 10 and 1l, while the resistors 8l and 82 are respectively connected in series in the grid circuits thereof.

With the contact member S5 in circuit-making engagement with the contact S6, it will be observed that the grid of the valve 'll is connected by way of the resistor 62, contact Eli, contact member' G5, and by a resistor 16 to the cathode. Thus, there will be applied to the grid a negative bias lhaving a magnitude determined by the current through the valve and by the resistance of the cathode resistor 1e. The resistors 1S and 80 have relatively high resistances-each may be of the order of 2 megohms7 while the resistors 8| and 32 may each be of the order of 200,000 ohms, with the respective cathode resistors 14 and 16 of the order of 2000 ohms. These circuit constants have been found satisfactory for electric valves or tubes of the 6J5 type. The negative bias applied to the grid of valve 1l greatly decreases the current flowing through it; hence, the relay @il remains in its illustrated de-energized position.

On the contrary, the grid of the valve 'l0 is effectively at cathode potential because of the connection thereto through the resistor 19. The resuit is that a substantial current flows through Cil the' valve 10 to energizel the relay 12 which opens its contacts. g

It will be recalled that the relay l5v was energized by the movement of the slide to close the slide-actuated switch 36. When this occurred, an energizing circuit was completed from the supply line 3| by way of the contacts of relay 58, contacts 9e of' relay 45, the operating coil of contactor lig, contacts 83 of the lower limit switch and to the other supply line 32. The contacter 69 closes to energize the motor to lower the electrode lil. The motor, through suitable speed reduction gearing, then operates to lower the electrode lil' at a relatively slow rate. As soon as the electrode l0' begins to move, the voltage applied to the dielectric material l2 rises and the output of the oscillator rises. As'soon as the output corresponds with the normal full-load rating thereof, the potential difference across the resistor I1 rises an amount sufficient to energize the sensitive device @fl for movement of its contact meinu ber G5 midway or contacts 55 and 51. As soon as the circuit is broken Vthrough the contact 66, the negative bias theretofore applied to the valve 1I disappears and the grid thereof is effectively connected by resistor Se to the cathode. In consequence, the relay 68 is immediately operated to its energized position. The opening of its contacts, of course, deenergizes the contactor 69 to stop the motor d8.

The system has now functioned automatically to bring the oscillator up to full load, or to any selected predetermined load. The selection of the desired load maybe readily made by relative adjustment between the resistor il and the conn tact i111.

As long as the electrical characteristics of the dielectric material l2 remain constant, the oscil lator will have a substantially constant output. It is characteristic of some dielectric materials that the electrical properties thereof will vary with temperature. Thus, if the dielectric material l2 is a preform of a phenolic molding compound, the power factor will increase as the temperature thereof rises. In consequence, the oscillator would deliver a greater power output to the material l2. The increased output is reec-ted by a rise in the anode current. The increased potential diiference, however, energizes the sensitive device 64 for operation of the contact member 65 to complete a circuit through the contact 61. Through this circuit, which includes resistor 8i, there is applied to the grid of valve 10, the negative bias from the cathode resistance 14 abruptly to decrease the current through the valve. Accordingly, the relay 12 is cle-energized to complete a circuit for the operating coil of the contacter 42. This circuit may be traced from the supply line 3l by way of conductors 43 and Se, contacts of relay 12, conductor 55, operating coil of the relay 42, and by way of the limit switch 136 to the other supply line 32. The motor l is thereby energized in a direction to raise electrode I0. It is raised until the anode current of the Avalve means I3 returns to its normal or predetermined value, at which point the sensitive device E4 operates to interrupt the gridbiasing circuit to the valve 10, thereby to energize the relay 12 and to de-energize the motor 2S.

Should the power factor of the dielectric material l2 decrease, or should the load decrease, it will be understood that the decreased voltage drop across the resistor l1 will cause the sensitive device Gil to complete a circuit by way of the contact B6 for cle-energization'ofthe relay t8.

In consequence, the motor 49 will be energized to lower the electrode I until full load has been again established on the oscillator including electric valve I3.

The foregoing operations are entirely automatic. They are carried out in an eilicient and reliable manner. The system will correctly function under a wide variety of conditions to maintain a predetermined, or the maximum permissible, power loading or" the oscillator and will insure maximum eiciency of operation of the valve means I3.

Now that the principles of the invention have been explained, it will be understood that the invention may be applied to oscillators of widely differing types.

The control system may be utilized to vary the position of the tap 23a which completes the connection from the conductor 23 to the inductor 22. Thus, by raising or lowering the tap 23a, .the voltage applied between the electrodes l0 :and may be increased or decreased. However, the adjustment of the spacing of the electrodes is the preferred method.

In heating preforms, and in many other applications of the invention, it is frequently desired to terminate the heating or treating cycle after a predetermined time interval. This may be conveniently done by including the time-delay relay 38 in the energizing circuit of the coil of relay 35. The operating coil of the relay 38 was energized by operation of the slide-actuated switch 36. The circuit may be traced from supply line 3i by way of switch 36, conductors t5 and 8l, operating coil of the timing relay 38, conductor S8, and by contacts 4o of relay 35 to the other supply line 32. rIhe timing relay 38 may be preset for any given time of operation. For example, after a heating interval of 15 seconds, this relay may operate to open its contacts to de-energize the relay 35. This relay in turn de-energizes the relay l5 which thereupon operates to close its contacts 44 to complete the energizing circuit for the motor-controlling contactor 42. In consequence, the motor 49 is energized to raise the electrode lo to its uppermost position, preparatory to the insertion oi additional plastic material between the electrodes |0 and for heating thereof. As soon as the next charge of material has been placed on the slide and moved between the electrodes, the switch 35 is operated from its illustrated position to initiate a new cycle oi' operations.

Referring to Fig. 2, there has been illustrated by the broken lines 89 a housing which may include the system as a whole and all of the circuit elements thereof. The housing may be provided with a drawer or slide 90 on which may rest a preform or other dielectric material l2, which is to be heated. It is to be observed that when this slide or drawer 9S is moved inwardly the end thereof engages one end ci an arm of the switch 35 to move the contacts from one circuit-closing position to the other. It may be further observed that the adjustable electrode l0 is mounted by one or more insulators 8| to an arm i2 carried to parallel links 93 and Se respectively pivoted at 95 and 96 from a frame 9i. The parallel links 93 and 913 maintain the electrode I0 parallel to the cooperating electrode for a relatively wide range of spacing therebetween. The link 94 carries a guide 9B for a cam 99 driven by the motor til, shown only in Fig. 1. It will be observed that rotation of the cam 99 in one direction or the other will raise or lower the electrode l0.

Instead of utilizing valves of the type gen-- erally known to those skilled in the art as vacuum tubes, there may be used tubes of the type sold under the trade name Thyratrons Referring to Fig. 3, a pair of grid-controlled arc rectiers or Thyratrons |00 and |0| of the tetrode type have been illustrated with their respective output circuits connected to include the operating coils of the relays 'l2 and 68. It will be observed, corresponding parts have been labelled with like reference characters. In some cases, letter subscripts have been added. Thus, the transformer 13a, energized from a suitable source of alternating current, has one secondary winding connected in the output circuits of the two "Thyratron valves |00 and |0|. This transformer has an additional secondary winding 13b connected in the input circuit to the valves; i. e., the winding 73h is connected to the cathodes thereof and to the juncture of a pair of resistors |02 and l 03, the opposite ends of which are connected to the control grids of the Thyratron valves |05 and The winding 13b applies to the respective control grids, a voltage having an instantaneous polarity which is negative with respect to the instantaneous positive polarity of voltage applied to the anode or output circuits. In consequence, the Thyratron valves |00 and iii! are normally biased to maintain them nonconductive. The resistors |02 and |03 have relatively high resistance values, of the order of one megohm, as compared with the resistance (of the order of one-tenth megohm) of resistors |04 and |05 connected in circuit with the control grids and the respective contacts 6l and 6G of the galvanometer 64. When the galvanometer coil 54 is energized to move its contact member 65 into engagement with the contact 6l, it will be observed that the control grid of the valve |09 is connected through the resistor |04 and the contact member 65 to ground. Since the cathodes are also connected to ground, the result is to decrease the voltage applied by the transformer winding 13b to the grid. The bias volt-1 age will be but a tenth of its former value and far below that necessary to maintain the Thyratron valve |00 in its non-conductive condition. Thus, the Thyratron valve |00 immedi ately "res or becomes conductive for passage 0f a substantially large current therethrough. This energizes the relay 'l2 to close the contacts 12a for energization, by way of the conductor 84, of the contactor 45, shown only in Fig. 1. At the same time, the relay l2 will open its contacts 12b connected in series with the operating coil of relay 68.

As soon as the power input to the load has been decreased, as already explained, the galvanometer 64 moves its contact member 65 out of engagement with the contact 8l, again to apply the higher value of negative bias potential to the control grid. The valve ed is rendered nonconductive to cle-energize the relay 12 which, of course, moves to its illustrated de-energized position.

In case of diminution of power input to the load l2, the galvanometer 64 completes a circuit through the contact 66 to reduce the negative bias on the control grid of valve |0|. In like manner, the operating coil of relay 68 is energized to close the contacts 68a for energization of the motor-controlling contactor 69. As soon as the power output of the oscillator has been increased to the desired value, the relay 68 is de-energized by the opening vof the circuit through the contact 66.

The galvanometer E4 is a relatively sensitive detector and operates from one contact-making position to the other upon a change of power of the order of 2%; i. e., if normal current for the oscillator is 99 milliamperes, the galvanometer wil-l complete .one circuit connection when the output current .is 9S milliamperes, and it will complete the other circuit Aconnection when the output current is 100 milliamperes. The actual current flowing through it may only change from 1.9.6 milliamperes to k2 milliamperes. `For currents of such a low order, it will be apparent that there may be considerable variation in the contact resistance of the galvanometer contacts, but that variation rwill have no effect upon the operation. The operation is such that as soon as the contact member touches either of contacts ed or El, one or the other of the Thyratron valves it!) or lili lires to produce positive and instantaneous energization of the corresponding relay.

Now that the principles of my invention have been fully explained, it will be understood that electric valves of any suitable type may be employed and that further modiiications may be made in the invention without departing from the true spirit and scope of the invention as set forth in the claims rThus, while the system in Fig. 1 operates to energize the relays upon opening of the galvanometer contacts, the system of Fig. 3 operates to energize the relays upon closure of those contacts. Either arrangement may be utilized, or in Fig. l, the relays may be connected for opening and closing of the circuit which includes the resistors le and Si), and instead of a motor e9, other driving means such as a hydraulic cylinder or air-engine may be utilized.

What is claimed is:

l. The combination with an electrical oscillator for generating high-frequency electrical energy, electrodes for applying said energy to a dielectric load, said oscillator including valve means, a combined output tank circuit where the capacitance of the dielectric load between said electrodes forms the capacity of the tank circuit, and a grid circuit energized from said tank circuit, of a motor for adjusting the position of one of said electrodes with respect to the other electrode, switching means for said motor for controlling the direction of adjustment of said one electrode relative to the other electrode, electric valves for controlling said switching means, a contact-making galvanometer responsive to change in the load on said oscillator forselectively controlling the conductivity of said electric valves and thereby controlling the operation of said switching means for producing operation of said motor in a direction to change the spacing and capacitance between said electrodes in a direction and to an extent to maintain at predetermined rate the high-frequency energy input to said load, relay means for completing a circuit for operation of said motor to separate one electrode from the other independently of operation of said sensitive means, a limit switch for bringing said electrode to standstill with said electrodes separated a predetermined distance one from the other, and means operable upon insertion of said dielectric load between said electrodes for energizing said relay means to open said last-named circuit to restore control of said motor to said electric valves and switching means associated therewith.

2. The combination with an electrical oscillator for generating high-frequency electrical energy, electrodes for applying said energy to a dielectric load, said oscillator including valve means, a source of anode supply, an output tank circuit where the capacitance of the dielectric load between said electrodes is included therein as the capacity of the tank circuit, and a grid circuit energized from said tank circuit, of a motor for adjust-ing the position of one or" said electrodes with respect to the other electrode, switching means for said motor for controlling the direction of adjustment of said one electrode relative to the other electrode, electric valves for controlling said switching means, a contactmaking galvanometer responsive to the load on said source of anode supply for selectively controlling the conductivity of said electric valves and thereby controlling.l the operation of said switching means for producing operation of said motor in a direction to change the spacing' and capacitance 1cetween said electrodes in a direction and to an extent to maintain at predetermined rate the high-frequency energy input to said load, relay means controlling the connection of said anode supply to said valve means and initially completing a circuit for operation of said motor to separate one electrode from the other independently of operation of said sensitive means, a limit switch for bringing said electrode to standstill with said electrodes separated a predetermined distance one from the other, and means operable upon insertion of said dielectric load between said electrodes for energizing said relay means to open said last-named circuit to restore control of said motor to said electric valves and for completing the connection of said source of anode supply to said valve means.

3. A high-frequency dielectric heating system comprising a self-excited electric oscillator for generating high-frequency electrical energy, said oscillator including valve means having a power supply input circuit and a frequency-determining output tank circuit including a pair of electrodes disposed in spaced heating relation to a dielectric load and forming the capacitance of said tank circuit, said load during heating impcsiing an inherently varying load demand upon said oscillator, structure for supporting one of said electrodes for movement relative to the other of said electrodes for varying the capacitance of said tank circuit by change or spacing between said electrodes to control the voltage applied to the dielectric load, reversible driving means for said structure, direction-controlling means for controlling said driving means for selective movement of said structure in one direction or the other to change the spacing and capacitance between said electrodes during heating of said dielectric load, said oscillator having a grid circuit energized from said tank circuit, an impedance connected in series with said power supply input circuit for developing a voltage which varies with change in said load demand imposed upon said oscillator, and means includa sensitive device connected in circuit with said impedance means for selectively controlling said direction-controlling means for moving one electrode toward and away from the other electrode to so vary the high-frequency voltage applied to said load as to maintain substantially constant the flow of current in said power supply input circuit ior delivery to said dielectric load of electrical energy at a substantially constant pre- WILFRID L. ATWOOD.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date Curtis et al June 23, 1925 Lindsay Apr. 14, 1931 Buyko Apr. 21, 1931 Van Benschoten May 30, 1933 Johnson Aug. 21, 1934 Hubbard Jan. 18, 1938 Johnson Mar. 1, 1938 Gohorel July 30, 1940 Hart et al Aug. 5, 1941 Number Number Name Date Mittelmann July 20, 1943 Zenner Nov. 30, 1943 Eastin Jan. 2, 1945 McWhirter Feb. 20, 1945 Mann et a1 Aug. 14, 1945 Gilbert Mar. 5, 1946 Reifel et al, Feb. 11, 1947 Gregory et a1. Feb. 18, 1947 Young et al Apr. l2, 1949 FOREIGN PATENTS Country Date Great Britain Dec. 2, 1935 Great Britain Sept. 28, 1943 OTHER REFERENCES Mittelmann, Lead Rematching in Electronic Heating, Electronics, February 1945, pages 110- 20 115, pages 114 and 115 relied on. 

